The present invention relates to the formation of products by hot isostatic pressing, and more particularly to a method for the rapid hot isostatic pressing of materials by the use of a liquid pressure medium.
Heretofore in the hot isostatic pressing of powder particles the pressure medium used has most commonly been an inert gas such as nitrogen, helium or argon. It is known, for example, that finely divided metal powders may be compacted into articles approaching 100% of theoretical density by placing a charge of powdered metal in a container, sealing, heating and evacuating the container, transferring the container to a pressure vessel or autoclave and subjecting the container and its contents to the application of high gas pressure. See, for example, U.S. Pat. No. 3,450,528 to Thompson.
It is also known that cold isostatic pressing may be accomplished by subjecting powder particles contained in a flexible mold to either gas or liquid pressure. See, for example, the description of prior art practices in U.S. Pat. No. 3,562,371 to Bush.
U.S. Pat. No. 3,577,635 to Bergman et al. discloses the isostatic compaction of powder bodies wherein the pressurizing medium is a liquid. That patent states that the container in which the powder particles are enclosed "may possibly also be heated to a certain extent," but there is no description given as to how powdered material heated, for example, to a compacting temperature of 2200.degree.F. may be isostatically pressurized.
Other examples of known methods and apparatus for the hot isostatic pressing of powder particles are disclosed in U.S. Pat. No. 3,427,011 to Boyer et al. The Boyer et al. patent refers repeatedly to the use of gas pressure as the pressurizing medium but also states that "liquids may be acceptable for particular operations" (col. 9, lines 17-18). However, no description is given as to how a liquid may be used for hot isostatic compaction. A detailed description of a typical gas pressure autoclave for the hot isostatic compaction of powder particles is given in the article entitled "Operation of High-Gas-Pressure Autoclaves at High Internal Temperatures" by Boyer et al., published Aug. 17, 1965 by Battelle Memorial Institute of Columbus, Ohio.
One of the problems encountered in the use of gas as the pressurizing medium in hot isostatic pressing is the time consumed in carrying out the process. A relatively long period of time is required adequately to heat the part to be compacted and to raise the pressure of the gaseous pressure medium to the desired level. Various techniques have been suggested for reducing the time it takes to hot isostatically press a powder part by means of a gaseous pressure medium. For example, U.S. Pat. No. 3,450,528 to Thompson discloses a process in which the powder metal charge is preheated before introducing it into the pressure vessel and subjecting it to high fluid pressure. However, Thompson states that in fluid-pressure vessels or "autoclaves" of the type used to practice his invention "high pressures are developed by the use of a gas such as helium."
U.S. Pat. No. 3,543,345 to Boyer discloses an apparatus for rapidly compacting articles from fully preheated powder metal charges by the use of a "high-pressure fluid such as helium gas" wherein a first pressure vessel used for compacting the part is pressurized by interconnecting it to a second or storage pressure vessel previously pressurized to a sufficiently high pressure so that when the interconnecting valve is opened the equilibrium pressure achieved in the system is sufficient for compaction of the powdered material. Boyer's system can function only if a gaseous pressure medium is used because it is dependent upon the expansion of the gas from the second or storage vessel to effect rapid pressurization of the first vessel. Boyer's system would not function if the pressurizing fluid were liquid because liquids are insufficiently compressible to pressurize a vessel using the technique described, i.e., the change in volume corresponding to the pressure difference involved would be too small to fill the first pressure vessel containing the powder metal charge.
In addition, while Thompson's and Boyer's improvements may reduce the time period in which the part to be compacted remains in the pressure vessel, they still entail the use of expensive gas compressors, and in the case of Boyer's interconnected pressure vessels, a second pressure vessel to store the compressed gas is essential.
Where gas is used as the pressurizing fluid, large, expensive multistage gas compressors may be required in order to achieve reasonable efficiency in reaching the desired pressure level because of the compressible nature of the fluid being handled. In addition, where gaseous pressure mediums are used, it is frequently necessary to include electrical heating elements within the pressure vessel in order either to raise the temperature of the part to be compacted to the desired level, or to maintain it at that level if the part is heated prior to being introduced into the pressure vessel. In such case, the fluid used as the pressure transmitting fluid will be in intimate contact with the electrical furnace elements and for this reason the fluid itself must be an electrical insulator.
Another problem encountered in connection with the use of a gaseous pressurizing medium is that of maintaining seals to prevent gas leakage. Control of leakage in gas pressurizing systems is difficult and costly. Also where gas is used as the pressurizing medium, high safety factors for the pressure vessels utilized are necessary because of the high compression required.